The major cell surface glycoprotein fibronectin is decreased after malignant transformation and is involved in cellular adhesion. We have investigated its mechanism of action. Polypeptide structural domains containing specific binding sites for collagen and the plasma membrane have been isolated and characterized biologically. High-affinity, separate binding sites for hyaluoronic acid and heparin were also characterized, and a third domain for heparin binding was identified. The role of fibronectin in normal differentiation was examined. Cartilage cell development involves the loss of cell surface fibronectin, apparently due to a loss of receptors. Vitamin A reverses these events. Maintaining fibonectin levels inhibits cartilage cell development. Our objectives will be to determine the composition and structure of the multiple active sites of fibronectin, and the mechanisms by which this major cell surface protein helps to regulate cell behavior and differentiation.